Sheridan field, in the Gulf Coastal Plain of Texas, is in west-central Colorado County about 85 mi west of Houston. The field is outstanding because of the large gas-condensate reserves found in the Wilcox section of Eocene age and unique for the trend because of the tremendous sandstone thickness and number of sandstone bodies found to be productive. The gross thickness of Wilcox sandstone ranges from 1,200 ft in the southwest part of the structure to 1,700 ft in the northeast part. Forty-three separate units have been recognized, of which 29 are producing or have produced in the past, and 5 others are indicated to be productive. Of the 29 productive Wilcox sandstone units, 8 are associated oil-gas reservoirs and the rest produce exclusively gas and condensate.

The structure, 14,900 acres in area, was first detected by use of reflection seismograph after a period of intensive exploration. In May 1940, Shell Oil Company completed the discovery well, the Plow Realty Company No. 1, as an oil-producing well in the upper Wilcox. Seismic interpretation, confirmed by later development, showed a simple, ovate anticline, uncomplicated by faulting, on the downthrown side of a normal, down-to-the-southeast fault which subsequently has been found to have a throw of approximately 150 ft. A total of 103 wells has been drilled in the field. Of the 95 which penetrated the Wilcox section, 86 were producing wells and 9 were dry holes. Of eight wells drilled exclusively to test the Yegua reservoir, six were producing wells and two were dry holes.

A gas-cycling plant was constructed just north of the central part of the field in 1944, and the first test runs were made in August 1945. The plant was built to cycle the Wilcox "L" and "P-5, Q" Sands. Subsequently, cycling programs have been undertaken for the "G-1" and "H" Sands. The program has now been discontinued for all except the "H" Sand. The "L" and "P-5, Q" are blanket sandstone bodies forming reservoirs of approximately 6,500 and 8,500 acres, respectively. The "H" Sand productive area is more than 7,000 acres, and that of the "G-1" Sand, which produces from a long, narrow stratigraphic trap on the west flank of the structure, is 1,850 acres.